**1. Formation of gypsum (CaSO _{4}·2H_{2}O)**

We demonstrated that very first stages of gypsum formation involved the formation of well-defined primary species <3 nm in size. With time, these primary species arranged into domains, as evidenced by the emergence of a clear structure factor contribution in SAXS patterns. As the system developed further, the scattering patterns became gradually dominated by large aggregates composed of the original primary species and this stage was followed by the growth of the particles within the aggregates, which we associated with their actual transformation and crystallization to gypsum. This latter stage we also evidenced through the auxiliary diffraction data (WAXS). We speculated that these species were anhydrous Ca-SO4 cores/clusters. However, from these SAXS/WAXS data we were unable to directly determine the atomic structure of the primary particles. Hence, we performed in situ total X-ray scattering measurements to understand the structural characteristics and transition of these elongated species to the final structure of gypsum. The derived pair distribution functions (PDFs) revealed the formation of elongated clusters within 40 seconds of the initiation of the reaction (i.e., mixing of individual aqueous ions). Based on the PDFs we derived plausible conformations of several cluster structures that will be discussed here. Our findings clearly demonstrate the power of multi-length-scale scattering methods for in situ determination of crystal nucleation, growth and crystallization mechanisms.

**2. Another brick in the wall - the actual structure factor for surface fractal aggregates**

In colloid physics and chemistry, fractal scaling concepts constitute an important formalism that provides a statistical description of their properties. The fractal nature of colloids can be experimentally quantified and validated using scattering techniques. Based on a combination of theoretical and experimental evidence two distinct scaling laws have been used to describe experimental observations: mass fractals and surface fractals. The core idea behind mass fractals stems from our need to statistically describe aggregation processes involving primary particles, and the mass fractal scaling represents the packing efficiency of an aggregate, which in turn depends on the type of aggregation. On the other hand, surface fractal scaling only relates to the perimeter of a particle (or aggregate of particles) and correlates with its specific surface area. For surface fractal objects the current concepts do not explicitly consider primary particles as possible building blocks, since their surface fractal dimensions cannot be associated with the internal packing efficiency. Hence, surface fractals in contrast to mass fractals, are essentially not considered as aggregates, but as single particles with rough surfaces.

Nevertheless, densely packed surface fractal aggregates form in systems with high local volume fractions of particles with very short diffusion lengths. This effectively means that particles have little space to move. However, there are no prior mathematical models, which describe scattering from such surface fractal aggregates and which would allow the subdivision between inter- and intraparticle interferences of such aggregates. We showed that by including a form factor function of the primary particles building the aggregate, a finite size of the surface fractal interfacial sub-surfaces can be derived from the ratio between the structure and the form factor term.. This formalism allows us to define both a finite specific surface area for fractal aggregates and the fraction of particle interfacial sub-surfaces at the perimeter of an aggregate. The derived surface fractal model is validated through an ab initio approach, which generates von Koch type “brick-in-a-wall” contour fractals. Moreover, we show that our model explains observed scattering intensities in in situ experiments where we followed the formation of gypsum (CaSO_{4}·2H_{2}O) from highly supersaturated solutions. Our model of densely packed “brick-in-a-wall” surface fractal aggregates may well be the key precursor step in the formation of different types of mosaic- and meso-crystals.

The making of bonds during the formation of inorganic solid phases from solution usually follows a series of complex steps. The fact that the emergent species are often structurally nano-particulate and in many cases unstable, dictates that ideally they must be characterised not just at length-scales < 100 nm but also as in situ as possible. To this effect, solution-based X-ray scattering methods constitute one of the most versatile tools in studying nano-structured materials as they form.

In this project we apply the scattering methods: small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS) and total scattering (with pair distribution function, PDF, analysis) to study the formation and transformation pathways of various mineral phases from solutions. Currently, our research activities are focused of the formation of calcium sulfate (gypsum, bassanite), phosphates (struvite), and various aluminosilcates (saponite, bentonite, geopolymer cements). Furthermore, we develop numerical and fundamental mathematical methods for the scattering data modeling and interpretation.

We are advanced users of X-ray producing synchrotron facilities such as Diamond Light Source (UK), European Synchrotron Radiation Facility, ESRF (France), or DESY (Hamburg). Through local collaborations we have also an access to laboratory-source instruments in the Berlin area.

**
Dr. Tomasz Stawski
**

Interface Geochemistry

Telegrafenberg

Building C, room 121

14473 Potsdam

tel. +49 331 288-27511

**
Dr. Rogier Besselink
**

Interface Geochemistry

Telegrafenberg

Building C, room 127

14473 Potsdam

tel. +49 331 288-27505

**
Dr. Jörn-Erik Hövelmann
**

Interface Geochemistry

Telegrafenberg

Building C, room 128

14473 Potsdam

tel. +49 331 288-28703

**
Prof. Dr. Liane G. Benning
**

Interface Geochemistry

Telegrafenberg

Building C, room 124

14473 Potsdam

tel. +49 331 288-28970